Spoiler: Inconsistency between Bahamut and the 3.56 reveals

[REPROBEAN_CHILD]

By now, it's confirmed that Omega was used to bring down Bahamut. Not only that, but Shinryu was believed to be "more powerful than Bahamut," said by Ilbred. But despite what we've seen from both Omega and Shinryu, I have a hard time believing they're greater than Bahamut in terms of power. I'm not sure how large Omega and Shinryu are compared to him, judging by how big they are compared to Belsar's Wall. Omega is at least bigger than Ultima Weapon and Shinryu seems larger than the first brood (Hreavelgar, Nidhigg, Tiamat) and smaller or equal to Midgardsormr. But neither seem anywhere close to the size of Bahamut, who was MASSIVE. He was much bigger than Limsa itself and flew across Eorzea in a matter of seconds. Bahamut's rampage also changed the shape of the continent. Coerthas's climate changed, The West Shroud was annihilated, massive clusters of corrupt crystals could be found throughout Eorzea, Carteneau is in ruins, and the landmarks from 1.0 were either massively changed or destroyed. The fight scene between Omega and Shinyru was awesome, but I have a hard time believing they're capable of being anywhere near close to being stronger than Bahamut. All they did was make an explosion of light. Bahamut nearly destroyed the whole continent.

[Belhi]

To be honest, I am not sure if Shinryu was as powerful but admittedly we only got to see a short battle. That said, what we did see would convinced me that Shinryu is quite capable of creating a calamity scale event. I mean he summoned a tidal wave in the middle of a desert that was as high as Bealsar's wall.

As for Bahamut, he was probably a lot weaker when he was taken down my Omega than when we saw him wake up. Bahamut have been a conduit for Aether for a long time by the time he broke out and caused the 7th calamity. We know that before beating him the Allagans weakened him by starving him of crystals to consume and using his dead as hosts for Voidsent to send against him. That war of attrition weakened him a lot before he was finally brought down.

[PArcher]

Simple answer: we don't know. There isn't a way to directly compare them at this point, except that Omega did capture Bahamut and threw him in Dalamud. Against Shinryu, Omega came to a draw.

Also, size doesn't directly correlate to power; how would we defeat anything if it did?

[zuzu-bq]

Omega was one of many things Allags had in their armies. It probably didn't single handly captured Bahamut but rather was only used to help doing the job on top of many other stuff during the war against Meracydia. This game story is full of inconsistencies but most of them have one big factor in common: the world's sotry told by eorzean historians is never truly accurate.

[Belhi]

The lore book states that the Allag wore down Bahamut due to the deal Xande cut with the Cloud of Darkness. Part of the deal had the Cloud of Darkness lend Xande command of her Voidsent for his armies. He used this to reanimate the vanquished enemies in Mericydia, turning the Voidsent possessed dead back against their former allies. This attrition slowly wore down Bahamut who had to continually expend power to destroy the possessed forms of his own allies and it finally reached the stage where the Dragons ran short of crystals which mean Bahamut was starved of power making him vulnerable to capture.

[Minsue]

The allags just attached a master ball to omega boom done.

[Torquil]

Bahamut's rampage also changed the shape of the continent. Coerthas's climate changed, The West Shroud was annihilated, massive clusters of corrupt crystals could be found throughout Eorzea, Carteneau is in ruins, and the landmarks from 1.0 were either massively changed or destroyed.

Keep in mind that a lot of what you cite here was caused more by the destruction of Dalamud than by Bahamut directly.

[Berethos]

Keep in mind that a lot of what you cite here was caused more by the destruction of Dalamud than by Bahamut directly.

Seems mostly even - Coerthas entering a sudden ice age, afaik, doesn't have a set moment for when it happened (other than during the events of the Calamity itself as a whole), so it could be a mix of both events completely disrupting the aether (though given that it's a disruption of aether I'm personally inclined to give the edge here to Bahamut's attacks). The West Shroud we can't directly see. The massive clusters usually surround giant pieces of Dalamud, so that's an easy one to factor in.

Ultimately, while a lot of Dalamud's pieces rained down (in a particularly violent manner) on numerous areas, Bahamut sent out an incredibly large amount of attacks himself, and just a few of those landing in one area seemed to hit pretty hard, which could certainly account for the West Shroud's destruction and the various violence the land itself went through that changed or destroyed the landmarks of 1.0.

With Carteneau itself, that looks like a 1-2-3 punch...first Dalamud's pieces that got flung straight down/sharp angle down, then Bahamut's stream of attacks, then his giant fireball. It was probably in ruins pretty much after the first one, but the next two certainly contributed to the overall destruction...

Which is probably true of most areas hit, to be fair, and why I'm inclined to say the destruction caused was a more even affair (though I'm personally giving the edge to Bahamut...rewatched it and that is a lot of attacks he rains down in a lot of areas).